When fish are kept in garden pools or ponds, and in ponds in open surroundings, mass-multiplication of planktonic algae gives rise to algal blooms. This phenomena represents a serious problem both aesthetically and biologically. In addition, the problem can also occur albeit with a lesser intensity in outdoor ponds that do not harbor fish, and even in indoor holding systems, e.g. aquariums. Efforts have and are being made to combat these algal blooms.
Besides the use of herbicides, flocculating agents have been used in recent years for removing the photo-plankton quasi-mechanically. In particular, flocculating agents based on monomeric and polymeric aluminum salts have proved to be very effective.
Despite the useful effect that was achieved, i.e. the clarification of the water, the treatment of biological waters and fish-keeping systems with solutions of aluminum salts entails a number of disadvantages such as harmful side effects, their severity varying according to the aluminum salt chosen.
Thus, for example, large or repeated doses of aluminum salts will reduce the carbonate hardness, or alkalinity, of the water being treated by taking-up available OH.sup.-- ions for the purpose of precipitating aluminum hydroxide.
Since dosing with aluminum salts entails the risk that the water will become insufficiently low in carbonate hardness or total alkalinity, the action taken to combat the algae will then become toxic for fish. This limits the single-application dose and the total number of doses that can be applied
Many experiments have shown that, in fresh waters with a carbonate hardness of approximately 3.degree. dH, or an alkalinity of approximately 1 mVal/l, or lower, there ceases to be any guarantee that the use of aluminum salts will be safe, and the occurrence of lethal toxic effects on fish becomes possible.
Even reducing the dosage of Al.sup.3+ ions, e.g. from 8 mg/l to 2-3 mg/l, fails to bring about any significant reduction in the toxicity, given that the abovementioned chemical conditions of hardness and alkalinity prevail in the water. Although not wishing to be bound by theory, the reason for this resides in the mechanism of the toxic effect.
At the typical pH values that generally prevail, for example between about 6.5 and 8.5, the Al .sup.3+ ion species present in solution no longer have toxic effects. However, so-called primary Al(OH).sub.3 flocs form at the beginning of the reaction in which the very sparingly soluble aluminum hydroxide is produced, and these primary flocs remain in colloidal solution.
These primary flocs, which are still positively charged, find their way onto the negatively charged gill tissues of the fish, and irritate the gills. The fish can suffocate as a result of an increase in the production of mucus by their gills.
These harmful effects become all the more powerful the longer the primary-floc formation phase lasts. Up to now, there was no way of selectively minimizing this vicious side effect of the Al(OH).sub.3 flocculation, as employed for eliminating algal blooms. Reliable protection was offered only by higher carbonate hardnesses, above 3.degree. dH, or alkalinities above 1 mVal/l. However, most garden ponds exhibit lower values, especially in years of high rainfall, and thus turn a flocculation treatment with aluminum salts into a considerable and incalculable risk as far as the fish are concerned.
Some aluminum-free flocculating agents, e.g. with organic polymers, would certainly be usable under soft-water conditions, but would give rather unsatisfactory results as regards clarification.
There thus remains a need for an efficient flocculating agent to remove algal blooms in garden pools or ponds and other interior systems harboring fish without the serious toxicity effects on fish.